The present invention relates to an accelerometer having a reasonable sensitivity at very elevated (ultrasonic) frequencies. The accelerometer in question comprises a housing which has a lower part in contact with the surface, the vibrations of which are to be determined, and a piezoelectric element associated with a seismic mass (inertial body). It is a purpose of the invention to provide an accelerometer having a predictable and hitherto unattained high sensitivity in the ultrasonic range of frequencies. It is a further purpose of the invention to provide an accelerometer which has a reduced sensitivity to transverse vibrations in the surface, the vibrations of which are to be measured.
It has been tried to cement the piezoelectric element directly to the vibrating surface and to cement the necessary seismic mass directly on top of the piezoelectric element. In this way an accelerometer of small weight has been obtained, which therefore is able to measure vibrations of relatively high frequencies. This simple solution is, however, not useable in practice since the vibrating surface always exhibits vibrations in several directions and not just perpendicular to the surface. The transverse vibrations in the surface develops spurious voltages in the piezoelectric elements. To avoid these, accelerometers having a very thick and stiff base have been constructed. Thus the bending of the surface to be measured will influence the piezoelectric element to a negligible extent only but instead the vibrating part of the accelerometer is made so heavy as to influence the vibrations to be measured, and furthermore the upper frequency limit is reduced appreciably. This is only acceptable when measuring low frequencies, e.g. in heavy machinery or buildings, but light constructions and high frequencies entail several difficulties. The light constructions are affected by the mass of the accelerometer, wherethrough the vibrations to be measured are distorted.
To preserve the upper frequency limit it is necessary at the same time to make the accelerometer small and therethrough light, and keeping a comparatively thick and stiff base which is heavy. A high resonance frequency is a prerequisite for high sensitivity at elevated frequencies since piezoelectric accelerometers can only be used below their resonance frequency. This would reguire the accelerometer to be of a low profile with a thin base, a thin piezoelectric element--everything contributing as little as possible to the mass. In this case, however, the sensitivity would suffer since it is proportional to the thickness of the piezoelectric disc and to the mass of the inertial body above it.
In particular when measuring acoustic emission where an indication of very high frequencies is desired, e.g. from 100 kHz to 2000 kHz, the problems are so severe that not only the base but also the seismic mass has to be entirely dispensed with. Accelerometers of this kind have been quite unable to perform a mesurement but can only give an indication of the occurence of vibrations.
Several designs of accelerometers have been published that try to avoid some of the problems outlined above, such as U.S. Pat. Nos. 3,145,311 and 3,394,586 and German Published Applications Nos. 1,005,758 and 1,228,448. Their size, however, gives rise to an error caused by the wavelength of the vibrations to be measured. At low frequencies where the wavelength is large compared to the dimensions of the accelerometer only errors caused by the bending of the base are present. At shorter wavelengths, from 2 to 5 times the dimensions of the accelerometer, an error is introduced, because some parts of the accelerometer are vibrating with a higher amplitude than others. At still shorter wavelengths some parts may vibrate in antiphase, thereby giving an indication of "little" or "no vibration". Another general difficulty in measuring vibrations of high frequency is coupling the vibrations from the vibrating surface to the accelerometer proper. The vibrations are to pass from the vibrating surface through the fixing means (e.g. cement, wax, threads, adhesive tape) to the base and through piezoelectric element to the seismic mass. Very often there exists an elasticity between the different layers which causes the seismic mass to vibrate differently from the vibrating surface. In particular, the contact between base and vibrating surface is critical and difficult. In other words: the correct fastening of the accelerometer is difficult to ensure. Even a threaded connection spanning polished surfaces may not be sufficient at the high frequencies involved.
It is a purpose of the invention to provide an accelerometer that does not suffer the disadvantages described above. This is obtained by dynamically isolating the accelerometer case from the vibrating surface, by carrying the base of the accelerometer, the piezoelectric element, and the seismic mass by one or a plurality of diaphragms which are peripherically fixed to the case.
Using the configuration described above makes possible both accurate control of the movements of the base perpendicular to the surface of the diaphragm and the sensing parts of the accelerometer become very light. The stiffness of the diaphragm causes a reduction of the accelerometer of ca. 10 times as compared to previously known accelerometers. As the diaphragm is fastened to to a case which is heavy as compared to the vibrating parts, this case may contain an amplifier and the necessary cable may be fastened to the case. This implies that the voltage sensitivity may be made quite normal--this depends upon the thickness of the piezoelectric element only but not upon it's area. When the area is small the capacity becomes small also and so necessitates an amplifier, and this is easily obtained in a construction according to the invention.
In one embodiment of the invention, the diaphragm is disposed between the base and the piezoelectric element. In this way it is made possible to weld the base of the accelerometer to a thin diaphragm.
In another embodiment of the invention, the piezoelectric element is disposed between the base and the diaphragm. This makes possible the use of the diaphragm's dynamic mass as a seismic mass, this serves to improve the useful upper range of the accelerometer.
In still another embodiment of the invention the base of the accelerometer is shaped as a spherical segment, e.g. obtained through the use of part of a steel sphere having a diameter of 1 millimeter and a height of 0.3 millimeter. Thus the contact area with the vibrating surface is made very small and the pressure similarly very large. In this way the base functions as if it were welded or brazed to the surface.
In still another embodiment of the invention the piezoelectric element and the seismic mass are disposed between two parallel diaphragms. Thus it is avoided that the assembly displays a rocking motion, thereby generating spurious signals.
In still another embodiment of the invention, the actual mechanical assembly is obtained by means of cement. This makes for an extremely lightweight construction ensuring a very high upper frequency limit.
In still another embodiment of the invention, the contact pressure between the base and the vibrating surface is so great that at least one of the contact surfaces suffers an elasic deformation. This contact pressure depends upon the weight of the accelerometer, on the resilience of the material that surrounds the base, and by the placement of the base in the case. By obtaining a sufficiently great pressure it is obtained that even high frequencies with associated high acceleration levels may be transmitted from the surface to the piezoelectric element without distortion.
In still another embodiment of the invention, the case on its underside carries a cushion of a flexible material, which cushion has a thickness such that the base of the accelerometer presses against the vibrating surface with a prescribed springiness and with a convex base. Thus it is obtained that the pressure of the base against the surface becomes a definite value which is mainly determined by the weight of the accelerometer minus a slight pressure from the cushion.
In still another embodiment of the invention, the convex base of the accelerometer projects freely from the underside of the case, so that the accelerometer may be held in one hand in contact with the vibrating surface. Thus it is obtained that one may manually increase the pressure of the base against the surface until a reliable contact has been obtained. This is a simple, inexpensive, and technically elegant solution.
In still another embodiment of the invention, the case of the accelerometer is fastened to an arm which is carried by a support that ensures that the base of the accelerometer is pressed against the vibrating surface with a prescribed force. This force is composed of the weight of the accelerometer and part of the arm, and furthermore, in case the arm is spring loaded, of a specific force exerted by the spring. Thus a fixed station for the accelerometer is obtained. This is desirable in case the accelerometer is to perform a series of measurements on essentially identical objects which may then be brought in contact with the base, one after the other.